1. Field of the Invention
The present invention relates to a method of operating a combined-cycle plant.
2. Discussion of Background
A modern combined-cycle plant according to the prior art is normally operated with the following control actions: the control members to be activated by the output or speed controller, namely compressor guide-row adjustment, intake-air preheating and steam injection with a steam quantity from the waste-heat steam generator, cause thermodynamic changes in the cycle, such as those of mass flow and pressure ratio. These changes in turn require an adaptation of the fuel quantities. This is done via appropriate control members which initiate fuel metering. If there is an intercooler in the compressor system, the cooling-water quantity can be changed via cooling-water metering in order to optimize the aerodynamic ratios of the sectional compressors.
Thus, for example, partial closing of the compressor guide rows and/or preheating of the intake air results in a decrease in the cycle air flow, which decrease brings about a decrease in the pressure ratio on account of the absorption principle of the turbine. In order to now prevent a temperature rise in the two combustion chambers, the fuel quantities must be reduced. This is expediently done by using the two parameters which can easily be recorded in a reliable manner--the turbine outlet temperature and the pressure at the outlet of the compressor system. However, direct recording of the hot-gas temperatures and the outlet temperature of the high-pressure turbine is actually problematic on account of the very high temperature level prevailing there.
Conversely, when the compressor guide rows are opened further, the two fuel quantities to the combustion chambers must be increased, which takes place via the same control mechanism as before. However, the increase in output thus achieved is based on an increased combustion-chamber and turbine mass flow, which involves an increase in the pressure ratio. The reduction brought about by this in the turbine outlet temperature on the low-pressure side only marginally reduces the steam production increased by the increasing mass flow, so that the useful output increases approximately in proportion to the increase in the air quantity delivered by the compressor. A greater increase in the operational output of the steam turbine is therefore not possible.